This invention relates generally to plasma etch systems and more particularly to methods and apparatus for detecting end-point when etching semiconductor wafers in plasma etch systems.
The processing of semiconductor wafers to create integrated circuits involves a sequence of processing steps which build up the multi-layer structures of the integrated circuits. These processing steps include the deposition of metals, dielectrics and semiconductor films; the creation of masks by lithography techniques; the doping of semiconductor layers by diffusion and/or ion implantation; and the etching of layers for selective or blanket material removal.
There is a continuing trend in the semiconductor manufacturing industry to increase the functionality and performance of integrated circuits by increasing the number of circuit elements within each integrated circuit chip. While in some cases this is accomplished by increasing the size of the integrated circuit chip, in most cases this is accomplished by reducing the size and increasing the density of the circuit elements.
As the size of circuit elements decreases and their density increases, the processing steps enumerated above become more critical. For example, when making integrated circuits with feature dimensions on the order of 3 microns, a layer on a semiconductor wafer can be etched by the immersion of the wafer in a acid bath. This so-called "wet etch" method is generally inadequate for feature sizes less than 2 microns because it is an isotropic process which etches equally well in all directions. With smaller feature sizes, the isotropic etching causes undesirable undercutting of the masking layer and possible destruction of the closely spaced circuit elements.
To address the need for a more anisotropic etch process various "dry etch" processes have been developed which do not require the immersion of the wafer into an etching liquid. The most common dry etch process, generically referred to as "plasma etching", uses reactive plasmas are formed to anisotropically etch films on semiconductor wafers. Conventional plasma etching forms the plasma between a radio-frequency (R.F.) powered anode and a grounded cathode. Another form of plasma etching, known as reactive ion etching (RIE), applies R.F. power to the cathode and typically allows the anode to electrically float. In either case, positive ions formed in the plasma are accelerated towards the wafer by the self-biased negative cathode to provide an effective anisotropic etch of the wafer.
It is important to be able to predict or measure when the desired layer of an integrated circuit has been etched through, i.e., when the etch process has reached "end-point", in order to prevent damage to the wafer caused by excessive over-etching. End-point detection is particularly crucial in plasma etching, because this detection tends to have much lower selectivity than comparable wet etching processes. For example, it has been reported in VLSI Electronics Microstructure Science.sup.1 that plasma etching of SiO.sub.2 over doped silicon or polysilicon may produce a selectivity of only 15:1 while nearly infinite selectivity may be achieved by wet etching in HF acid solutions. FNT .sup.1 VLSI Electronics Microstructure Science, Volume 8, 1984, "Plasma Processing for VLSI", editors Norman G. Einsbrush and Dale M. Brown, Academic Press, Inc., New York, N.Y.
The aforementioned VLSI Electronic Microstructure Science reference provides a good overview of end-point detection methods for plasma etch systems on pages 434-445. Briefly, end-point detection methods either monitor the emission spectra of the plasma, the surface layer of the wafer, or one of the operating parameters of the plasma etch system itself. Those end-point detection methods which monitor one of the operating parameters of the plasma etch system fall into two general categories, namely, those methods which monitor reaction chamber pressure and those methods which monitor the impedance of the high-frequency plasma.
Several examples of impedance matching monitoring systems are given in VLSI Electronic Microstructure Science, supra. For example, VLSI Electronics Microstructure Science reports that Ukai et al..sup.2 discovered that there is a significant impedance change in the plasma during the reactive ion etch (RIE) of aluminum with CCl.sub.4. Similar observations were made for the etching of Si.sub.3 N.sub.4 or polysilicon with a CF.sub.4 O.sub.2 plasma. A comparison of the impedance changes in the plasma during etching with optical emission end-point detection methods indicates that the impedance matching method is an sensitive as optical emission monitoring in determining end-point. FNT .sup.2 Journal of Vacuum Science Technology, 16, March/April 1979, pp 385-387, "End-point determination of aluminum reactive ion etching by discharge impedance monitoring", K. Ukai and K. Hanazawa
Ukai et al., supra, monitored the impedance of the plasma by monitoring the cathode voltage during the RIE etch process. The change in monitored impedance in the Ukai et al, process was reported to be caused by the physical or chemical reactions in the plasma discharge. See Ukai et al., page 385, column 1. As a result, Ukai et al, can only detect end-point when the concentration of material from the layer being etched in the plasma discharge changes substantially, i.e. after at least partial etch-through of the layer.
A problem encountered with plasma etching is that some materials, such as the silicon substrate itself, can become damaged by the plasma even after a brief exposure. Therefore, methods such as those described in VLSI Electronics Microstructure Science and Ukai et al. to determine the termination of an etching process are inadequate for plasma-sensitive substrates such as silicon and polysilicon because by the time these methods detect end-point the substrate may have already been damaged. It would therefore be extremely desirable to have a method which detects imminent end-point, i.e. a method for determining when the layer being etched is just about to be etched-through.